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November 1998 Quarterly Groundwater Monitoring Report
Section 3 - Water Level Trends
Water level measurements are taken at CSSA as part of the quarterly sampling protocol. All wells are measured with an electric water level indicator (e-line) except Well 10, which is measured by the air-line method. Water well upgrades were performed on CSSA Wells 1 and 11 on 7-13 May 1997, and on Well 9 on 17-20 November 1997. The upgrades included the installation of a PVC e-line measuring tube in both wells. Prior to the upgrades, Well 11 water levels were measured by the air-line method.
CSSA typically ceases pumping 48 hours prior to quarterly monitoring provided this cessation does not conflict with critical base water needs. The duration required for full water table recovery following pumping has not yet been determined. The impact of pumping by adjacent subdivision water wells upon the water table at CSSA is also unknown. Based on quarterly potentiometric maps of CSSA, groundwater flow direction fluctuates from southeast to southwest at gradients of 0.003 to 0.1 foot per foot (Parsons ES, 1996). Prior groundwater flow direction and rates are employed herein for comparison with those observed in the October-December 1997 and 1998 quarters.
3.1 - Basewide Flow Direction and Gradient
Groundwater potentiometric elevations using e-line and air-line measurements from 21 October 1997 are depicted in Figure 1, from 22 January 1998 in Figure 2 and from 6 November 1998 in Figure 3. To meet CSSA water supply needs, pumping was performed up to 48 hours prior to water level measurement. The water table elevations at Wells 9, 10, 11 and 1 likely represent the influence of this pumping. Groundwater elevations increased between January 1997 and October 1997, decreased from October 1997 to January 1998, and increased again from January 1998 to November 1998. The average increase in groundwater elevations from January 1997 to October 1997 was 105.4 feet, the average decrease from October 1997 to January 1998 was 43.4 feet, and the average increase January 1998 to November 1998 was 113.5 feet. The total average rise in groundwater elevations from January 1997 to November 1998 was 175.5 feet. The rise is predominantly the result of extended rainfall in the latter parts of 1997 and 1998. Historical groundwater elevations from October 1992 through December 1998 are shown in Table 1.
Groundwater flow direction varied from west to south between January 1997 and October 1997, and remained flowing in a southerly direction through November 1998. Drawdown in groundwater elevations surrounding production Wells 9, 10 and 11 is observed on Figures 1, 2 and 3 due to pumping. The groundwater elevation data for these three wells was not used in determining groundwater flow direction or hydraulic gradient. The potentiometric surface maps indicate that the groundwater hydraulic gradient was to the south/southeast and varied from 0.002 feet per foot (ft per ft) in October 1997, to 0.003 ft per ft in January 1998, to 0.013 ft per ft in November 1998.
3.2 - Well 16 Water Level and Transducer Data
A meteorological station was installed just east of Well 16 and a pressure transducer was installed downhole in Well 16 on 22 and 23 August 1995. This was done to aid in determining the influence of local precipitation upon water table response using site specific data. The transducer malfunctioned and no viable data was collected from the transducer from April 1998. The first meteorological station, due to poor performance and an inability to meet CSSA standards, was replaced with a solar powered meteorological station the first week of April 1998. Precipitation data between 1 April 1998 and 31 December 1998 collected from the new meteorological station is shown in Figure 4. As discussed earlier, the large amount of rainfall received in the fall of 1998 prompted CSSA to request the 6 November 1998 groundwater sampling event.
A new transducer was installed in Well 16 and water table data collection began on January 11, 1999. This data and the precipitation data will be presented in the next quarterly report. As measured by e-line, a 192.77 foot rise in water level occurred between 6 January 1997 and 20 November 1998. Approximately 62.12 feet of this rise occurred between 20 October 1998 and 20 November 1998. This rise coincided with an extended period of heavy precipitation as measured at the Well 16 meteorological station and observed in the field by CSSA personnel. A single rainfall event of 12.29 inches was recorded at the meteorological station at CSSA on 17 October 1998. This rainfall event was verified through the National Weather Service from San Antonio, Texas, which recorded 11.26 inches of rain on the same day.